1800 Amp Harmonic filter reactor - May be single phase or three phase units. Mostly used together with capacitors to filter certain harmonic frequencies. Usually iron cored.

Harmonics are generated by non-linear loads. Electrical systems can comprise several loads which generate harmonics, such a variable speed drives, welding machines, computers etc.
Harmonic currents can result in increased losses and heat in electrical components, which can cause premature  aging and failure if the components are not selected or designed to handle these harmonic currents. In an attempt to reduce the effect of harmonic currents on the power system, electric utilities often impose financial penalties when a certain level of harmonic distortion on the system has been exceeded.
Harmonic currents may be reduced or eliminated by the use of filters. The harmonic filters, consist of reactors and capacitors, and may be tuned to the harmonic frequency which needs to be eliminated. There may be several filter circuits in order to eliminate harmonics of different frequencies.